The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Approximately 100,000 of the 2.5 million people in the United States who suffer from epilepsy are serious candidates for surgical treatment. Interventional neurosurgery usually requires laborious, invasive pinpoint mapping of cortical function (local field and action potentials) to distinguish eloquent from dormant brain tissue prior to resection. Whereas EEG recording from many electrodes (>100) on the skull generally provides poor spatial resolution (>1 cm) of synchronous neural activity, subdural and depth electrodes dramatically improve spatial precision at the cost of invasiveness. Based on the acoustoelectric effect (AE effect or AEE), an interaction between local pressure and density, we assess whether ultrasound can be used to improve contrast and resolution of traditional electrical recording. This approach potentially improves spatial selectivity by reducing the source size to the position where only ultrasound and current wavefields intersect. An acoustic pressure wave P traveling in a biologic medium induces a local change in conductivity (given byd(ρ)/ρ=K1(dP)  (1)with K1 an interaction constant on the order of 0.01-0.1% per MPa in physiologic saline [2,3]. When P intersects a current field i in a uniform conducting medium, the change in conductivity leads to a voltage modulation V between two recording electrodes of resistance R0VAE(t)=K2iR0P(t)  (2)
Thus, the phase and magnitude of the induced AE voltage is proportional to the ultrasound pressure and applied current. Jossinet et al. have provided a more complete analysis of the AEE in an electrolyte solution. Although AEE was first reported by Fox et al. and was initially used to characterize colloids in solution, others have recently proposed applications of AE to medicine and biology. For example, acoustoelectric tomography has been proposed for high resolution electrical impedance imaging of breast tissue. The present disclosure relates to a system capable of measuring VAE (t) in an electrically active living tissue environment that reflects the applied passive current and pressure fields. A need therefore exists to develop sensitive methods to characterize and map electrical current in living electrically active tissue and provide additional spatial resolution using a minimally invasive technique that is cost effective over the existing electrical current mapping technologies.